The present invention relates to a method of manufacturing a hand for an analog electronic timepiece, that is, a hand (especially, a second hand) for an analog electronic timepiece in which a long hand part for indicating time, a mounting part to be mounted to a hand shaft, and a short hand part extending to the opposite side to the long hand part with respect to the mounting part are integrally formed, and a weight part is provided on the short hand part.
First, the basic structure of a three-hand analog electronic timepiece will be explained with FIG. 7. In a typical three-hand analog electronic timepiece, torque generated by a step motor 60 composed of a rotor 60a, a stator 60b, and a coil 60c is transmitted from the rotor 60a to a fifth wheel 61, a second wheel 62, a third wheel 63, a center wheel 64 until an hour wheel 66 in order through pinions 71 to 75 and a minute wheel not shown respectively with being reduced to a predetermined rotation speed.
Further, a second hand 69, a minute hand 68, and an hour hand 67 are mounted on the second wheel 62, the center wheel 64, and the hour wheel 66 respectively to fit thereon through coaxial hand shafts respectively, so that time is indicated by the hands. It should be noted that the second hand 69, the minute hand 68, and the hour hand 67 are collectively called the hands. Incidentally, numeral 51 denotes a main plate, and numeral 52 denotes a train wheel bridge.
While the hands are standing still, each of the hands is held by the holding energy possessed by the step motor 60 to prevent a hand-skip phenomenon from occurring on receiving an external impact.
On the other hand, while the hands are moving, the step motor 60 generates driving energy which exceeds the holding energy so as to drive the hands.
FIG. 8 is a perspective view showing an example of the shape of a conventional typical second hand. This second hand 69 is composed of a long hand part 69a for indicating time, a mounting part 69b to be mounted to a second hand shaft, and a short hand part 69c extending to the opposite side to the long hand part 69a with respect to the mounting part 69b, and these parts are normally made of the same material into the same thickness.
A cylindrical fitting member 50 is firmly secured to the mounting part 69b, and the fitting member 50 fits over the second hand shaft 62a which is provided integrally with the second wheel 62, whereby the second hand 69 is mounted to the second wheel 62. Thus, the long hand part 69a indicates time (second). The minute hand 68 and the hour hand 67 are the same in basic shape.
Incidentally, an inconvenience of replacement of batteries once per several years is pointed out in a recent electronic timepiece, and it is desired that replacement of batteries is made unnecessary. As a measure to that, increase in capacity of a battery and reduction in power consumption are considered, but upsizing of the battery for increasing the capacity thereof can not be expected because of limitation in size of a wristwatch. Furthermore, the reduction in power consumption already comes to a limit though improvements in electromechanical conversion efficiency of the step motor have been made mainly by reducing the size of a magnet through enhancement of the power thereof or by optimizing a driving wave form or the like, and therefore more drastic reduction in power consumption can not be expected through the use of conventional methods.
Moreover, in the analog electronic timepiece, holding energy for holding the hands is generally required in order to prevent a hand-skip phenomenon caused by rotational energy, disturbance energy, generated by an external impact received during the standing still. As the holding energy, holding energy called magnetic potential (resistance force against moving from a still point) possessed by the step motor is normally used, which needs to be set at a value larger than a value of the disturbance energy generated by the external impact.
The magnitude of the disturbance energy, to which the moment of a rotating body composed of each hand, and the gear, pinion, and shaft to which the hand is mounted is related, is generally determined by the moment of the long hand part and the short hand part of the hand in relation to the rotational axis.
However, in a typical second hand used in the conventional analog electronic timepiece, since the long hand part 69a and the short hand part 69c are the same in thickness and different in length as is clear from FIG. 8, the moments of both parts with respect to a rotational axis line 70 are apparently unbalanced. Therefore, the second hand has some moment, resulting in occurrence of disturbance energy by an external impact.
The longer the long hand part becomes as the hand becomes larger as especially in a wristwatch for men, the greater the moment becomes, and separately the moment becomes larger in a designed hand which is designed differently in shape from a viewpoint of decoration, resulting in increased disturbance energy.
On the other hand, during driving the hand, the step motor generates driving energy exceeding the holding energy value which is set to be larger than the disturbance energy value so as to drive the hand.
The driving energy value here is a resultant value obtained by subtracting the holding energy value from a value of the whole energy generated by the step motor. In other words, the driving energy value is a value of effective energy, which rotationally moves the hand only a fixed angle in a predetermined period of time, beyond the value of the holding energy possessed by the step motor.
Therefore, it is apparent that if the holding energy value is decreased, energy which is consumed to exceed the holding energy value decreases and the whole energy required to drive the hand also decreases. In other words, it can be said that decreasing the holding energy value is effective to reduce power consumption.
As described above, however, the holding energy value can not be made small enough from the viewpoint of holding hand. Conversely, when low power consumption is achieved by decreasing the holding value, there is a problem that the disturbance energy value is larger than the holding energy value, whereby the hand can not be held adequately though the hand can move, bringing about a hand-skip phenomenon.
Hence, it is proposed that, for example, a weight is added to the short hand part 69c of the second hand 69 shown in FIG. 8 to reduce imbalance between the moments of the long hand part 69a and the short hand part 69c with respect to the rotational axis line 70 so as to decrease the disturbance energy value. This can also decrease the holding energy value, whereby the energy, which is consumed for the driving energy by the step motor to exceed the holding energy during the driving of the hand, is also decreased, making it possible to reduce the power consumption.
In order to manufacture the hand to which the weight is added as above, the whole hand has been conventionally formed by being punched out from a metal thin sheet material with a uniform thickness, and thereafter a small weight is bonded to an under face of the short hand part with an adhesive.
However, workability of the work of bonding the weight to the short hand part of a small hand such as a second hand of a wristwatch is poor, and thus it takes a great deal of time. Further, the weight is susceptible to being detached from the hand in use for a long time if the bonding is inadequate.
This invention is made to solve the above-described problems, and its object is to make it possible to manufacture easily and securely a hand for an analog electronic timepiece in which a weight part is provided on a short hand part and also to eliminate the danger for the weight to be detached from the hand.
To attain the above objects, the invention proposes the following first to third manufacturing methods as a method of manufacturing a hand for an analog electronic timepiece in which a long hand part for indicating time, a mounting part to be mounted to a hand shaft, and a short hand part extending to the opposite side to the long hand part with respect to the mounting part are integrally formed and a weight part is provided on the short hand part.
In the first manufacturing method according to the invention, the following first and second steps are performed in order:
a first step of making a part in a hand forming part forming the long hand part and the mounting part of the hand thinner in thickness to form a thin wall part in a thin sheet base material including the hand forming part; and
a second step of punching out the hand in a final shape from the base material to form the long hand part and the mounting part with the thin wall part of the hand forming part and the short hand part with a part except for the thin wall part, respectively,
wherein in the first step, it is preferable that the thin wall part is formed by press working.
In this case, if before the first step, a step of punching out a pair of parallel rectangular windows on both sides of the hand forming part of the base material is performed, the windows becomes places to which the material escapes when the thin wall part is formed by press working, facilitating the press working.
Further, in the first step, it is also suitable that the thin wall part is formed by press working at the part forming the long hand part and the mounting part of the hand in the hand forming part of the base material, and a thick wall part which is thicker than the original thickness is formed by coining at the part adjacent to the thin wall part forming the short hand part, respectively. In that case, in the second step, the hand in a final shape is punched out to form the long hand part and the mounting part with the thin wall part of the hand forming part and the short hand part with the thick wall part, respectively.
Incidentally, it is preferable that a plate material of brass or aluminum is used as the base material.
In the second manufacturing method according to the invention, the following first to third steps are performed in order:
a first step of punching out a window in a thin sheet base material having the same thickness as that of the long hand part and the mounting part of the hand to surround three sides of the weight part of the short hand part;
a second step of bending up the part of the base material forming the weight part and folding it to overlap on the part forming the short hand part; and
a third step of punching out the hand in a final shape from the base material to form the long hand part and the mounting part with the part forming the long hand part and the mounting part and the short hand part with the part forming the short hand part on which the part forming the weight part is overlapped, respectively.
In this case, following the first step, a step of overlapping a plate-shaped piece on a region including the part forming the short hand part of the hand on the base material except for the part forming the long hand part and the mounting part of the hand and the part forming the weight part is further performed, and in the second step, the part forming the weight part of the base material is folded to overlap on the part forming the short hand part with the plate-shaped piece interposed therebetween.
Further, it is preferable that a plate-shaped piece having the same width as that of the base material is used as the plate-shaped piece, and a window which corresponds to the part forming the weight part of the base material and the window surrounding the three sides thereof is formed in the plate-shaped piece in advance.
If a plate material with a density higher than that of the base material is used as the plate-shaped piece, a weight part with a great mass can be formed even with a small short hand part.
It is preferable that, for example, a plate material of brass or aluminum is used as the base material, and a plate material of tantalum is used as the plate-shaped piece.
In the third manufacturing method according to the invention, the following first and second steps are performed in order:
a first step of joining a weight forming member to a part of the surface of a thin sheet base material having the same thickness as that of the long hand part and the mounting part of the hand; and
a second step of punching out the hand in a final shape from the base material to form the long hand part and the mounting part of the hand with a part to which the weight forming member is not joined and the short hand part provided with the weight part with the part to which the weight forming member is joined, respectively.
If a material with a density higher than that of the base material is used as the weight forming member, a weight part with a great mass can be formed even with a small short hand part.
It is preferable that, for example, a plate material of brass or aluminum is used as the base material, and a tantalum material is used as the weight forming member.